Nitrate-Dependent Ferrous Iron Oxidation by Anaerobic Ammonium Oxidation (Anammox) Bacteria

Abstract

ABSTRACTWe examined nitrate-dependent Fe2+oxidation mediated by anaerobic ammonium oxidation (anammox) bacteria. Enrichment cultures of “CandidatusBrocadia sinica” anaerobically oxidized Fe2+and reduced NO3−to nitrogen gas at rates of 3.7 ± 0.2 and 1.3 ± 0.1 (mean ± standard deviation [SD]) nmol mg protein−1min−1, respectively (37°C and pH 7.3). This nitrate reduction rate is an order of magnitude lower than the anammox activity of “Ca. Brocadia sinica” (10 to 75 nmol NH4+mg protein−1min−1). A15N tracer experiment demonstrated that coupling of nitrate-dependent Fe2+oxidation and the anammox reaction was responsible for producing nitrogen gas from NO3−by “Ca. Brocadia sinica.” The activities of nitrate-dependent Fe2+oxidation were dependent on temperature and pH, and the highest activities were seen at temperatures of 30 to 45°C and pHs ranging from 5.9 to 9.8. The mean half-saturation constant for NO3−± SD of “Ca. Brocadia sinica” was determined to be 51 ± 21 μM. Nitrate-dependent Fe2+oxidation was further demonstrated by another anammox bacterium, “CandidatusScalindua sp.,” whose rates of Fe2+oxidation and NO3−reduction were 4.7 ± 0.59 and 1.45 ± 0.05 nmol mg protein−1min−1, respectively (20°C and pH 7.3). Co-occurrence of nitrate-dependent Fe2+oxidation and the anammox reaction decreased the molar ratios of consumed NO2−to consumed NH4+(ΔNO2−/ΔNH4+) and produced NO3−to consumed NH4+(ΔNO3−/ΔNH4+). These reactions are preferable to the application of anammox processes for wastewater treatment.